The present invention relates to an engine-turbocharger unit for a vehicle, in particular an industrial vehicle, including an internal combustion engine, e.g. a diesel engine, and a turbocharger for supercharging the engine.
The engine includes a number of cylinders, each associated with at least an intake valve and at least an exhaust valve for selectively connecting the cylinder to an intake manifold and an exhaust manifold respectively.
The turbocharger comprises a turbine having an inlet connected to the exhaust manifold and therefore powered by the exhaust gas of the engine; and a compressor driven by the turbine and having an inlet connected to an air intake circuit, and an outlet connected to the intake manifold of the engine.
As is known, in recent years IC engines have been devised featuring a decompression brake device or exhaust brake based on the principle of dissipating the compression energy originating in the engine cylinders to generate braking power, e.g. by opening the exhaust valves of the engine cylinders at the end of the compression stroke. The efficiency of this solution is increased by the supercharge provided by the turbocharger, which increases compression work and, hence, braking power.
Being a centrifugal machine, however, the contribution of the turbocharger decreases alongside a reduction in engine speed, so that, when the exhaust brake is activated at low engine speed, the decompression braking effect is poor, on account of the low intake pressure of the engine (boost pressure) and, hence, the small amount of compression energy dissipated.
To improve exhaust brake efficiency, turbochargers are commonly used featuring variable-geometry turbines (VGTs), i.e. turbines comprising a bladed rotor and a variable geometry nozzle surrounding the rotor and adjustable so as to vary the nozzle throat area of the turbine.
The variable geometry nozzle is adjusted as a function of the operating condition of the engine to permit continuous boost pressure control, i.e. to control the pressure of the output air from the compressor, which is substantially equal to the air pressure in the engine intake manifold.
More specifically, at low engine speed, the variable geometry nozzle is maintained in a closed attitude to minimize the nozzle throat area in the turbine and so increase the speed of the exhaust gas, so that the turbine rotor rotates faster to increase boost pressure.
Best braking power could be theoretically achieved by maintaining, at the same time, both boost pressure and the exhaust manifold pressure at their upper limit, i.e. as high as the engine mechanically allows.
However, at high engine speed, it is necessary to xe2x80x9copenxe2x80x9d the variable geometry nozzle, i.e. to increase the nozzle throat area, in order not to exceed the boost pressure an/or the turbocharger speed limit, which cannot be achieved without the natural consequence of the exhaust manifold pressure falling far below its target.
This happens because in known engine-turbocharger units boost pressure, turbocharger speed and exhaust manifold pressure (xe2x80x9cpre-turbinexe2x80x9d pressure) are inherently interrelated: opening the variable geometry nozzle leads to a decrease of pre-turbine pressure, due to a lower degree of throttling; this leads to a lower gas speed through the turbine, which results in lower turbine power and therefore lower speed of the turbocharger. Boost pressure is thereby reduced.
In conclusion, limiting the turbocharger speed and/or the boost pressure by increasing the turbine nozzle throat area inevitably reduces pre-turbine pressure and, hence, exhaust cycle work and its contribution to braking power.
In other words, to maintain a maximum permissible boost pressure or a maximum permissible turbocharger speed at high engine speed, the variable geometry nozzle throat may have to be so increased that the pre-turbine pressure and hence braking power fall below the desired level.
Engine-turbocharger units are also known in which the exhaust brake is not controlled by a variable geometry turbine, but by a throttle valve on the exhaust duct, downstream of the (fixed geometry) turbine exit, to generate a counterpressure in the manifold as a function of the throttle valve opening, so that, as the exhaust gas is expelled, the engine must work to overcome the counterpressure. When the decompression brake device is activated, the throttle valve is kept practically fully closed to obtain a high gas pressure in the exhaust manifold. On account of the extent to which the throttle valve is closed, such units are characterised by low gas flow and, hence, a low level of activity of the turbocharger, i.e. low turbine power, thus resulting in reduced air supply to the engine, an increase in temperature in the combustion chambers, and a reduction in the maximum braking power obtainable.
It is an object of the present invention to provide an engine-turbocharger unit for a vehicle, in particular an industrial vehicle, designed to eliminate the aforementioned drawbacks typically associated with known engine-turbocharger units.
According to the present invention, there is provided an engine-turbocharger unit for a motor vehicle comprising:
an internal combustion engine having a decompression brake device;
a variable geometry turbocharger comprising a turbine with a variable geometry nozzle connected to an exhaust manifold of said engine, said variable geometry nozzle enabling the control of turbine power according to operating conditions of said engine by commanding said variable geometry nozzle within its design-given geometrical restrictions, and a compressor driven by said turbine and having an outlet connected to an intake manifold of said engine;
characterised by comprising:
a throttling device located on an outlet side of said turbine and settable between a maximum-opening attitude and a minimum-opening attitude; and
control means for controlling both said variable geometry nozzle and said throttle device according to operating conditions of said engine, thereby allowing independent control of the exhaust manifold pressure of the combustion engine and of the expansion ratio of said turbine.